1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine and more particularly to an air-fuel ratio control system for an internal combustion engine equipped with a three way catalytic converter for the purification of the exhaust emissions.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air-fuel ratio of the engine is controlled around a stoichiometric value (14.7:1 for example) in accordance with the composition of exhaust gas and the operating conditions of the engine since an optimum operation of the three-way catalytic converter is enabled at the stoichiometric air-fuel ratio. The so called air intake side secondary air supply system is an example of the air-fuel ratio control system of this type. As an example, Japanese Patent Application No. 57-217548 of the present applicant discloses an air-fuel ratio control system of the air intake side secondary air supply system type. In this system, a solenoid valve is disposed in an air intake side secondary air passage leading to the intake manifold downstream from the throttle valve. The solenoid valve is open/close controlled in accordance with an output signal of an oxygen sensor disposed in the exhaust pipe. Further, a vacuum responsive air control valve is provided in the air intake side secondary air supply passage and the amount of the air flowing therethrough is controlled by supplying the air control valve with a control vacuum which is obtained by combining a vacuum in the intake manifold adjacent to or downstream of the throttle valve and a vacuum of the inside of the venturi, i.e., the so called venturi vacuum. Thus the feedback control of the air-fuel ratio is performed by controlling the amount of the air intake side secondary air.
In the air-fuel ratio control system of this type, the temperature of the engine coolant is detected and the above mentioned feedback control is started when the engine coolant temperature has reached a predetermined threshold value. When the engine coolant temperature is low, such as in a period after the cold start of the engine, the frictional resistance of the engine is relatively large and the poor atomization of fuel is likely to result in unstable engine operation. Further, the oxygen sensor is not activated in this condition. Therefore, the feedback control is stopped and the open loop control is selected to control the air-fuel ratio to the rich side. In an engine provided with an auxiliary fuel supply system such as an economizer, in order to improve the driveability and to increase the engine power, the operation of the auxiliary fuel supply system is stopped during the air-fuel ratio feedback control, and the auxiliary fuel supply system is operated to enrich the air-fuel mixture when the feedback air-fuel ratio control is stopped.
Thus, generally the open loop control is selected during a cold start period so that the air-fuel ratio of the mixture is controlled to the rich side, to stabilize the engine operation. The other reason for controlling the air-fuel ratio to the rich side during the cold start period is that the air-fuel ratio of the mixture is dependent on the density of the intake air and the air-fuel ratio of the mixture becomes lean during cold operation of the engine because of the high density of the cold intake air.
However, since the selection between the closed loop control and the open loop control is performed according to the engine coolant temperature in the conventional system, there was a problem that the engine operation becomes unstable in the event that the intake air temperature remains low even after the engine coolant temperature has reached the predetermined threshold level for starting the closed loop air-fuel ratio control.
Further, if a high threshold level of the engine coolant temperature is set for the determination of the start of the closed loop control, there will be another problem that an excessively long time is required before the start of the closed loop control, which causes the generation of undesirable emissions and a poor fuel economy.
In addition, if an automatic choke is provided to the engine having the above construction, there can be further problem that the venturi vacuum becomes excessively high if the choke valve remains closed due to the cold intake air temperature when the engine coolant temperature has reached the level to start the feedback control. Such an increase of the venturi vacuum will result in an excessive increase of the magnitude of a control vacuum to be used for the control of the air control valve and further results in an over lean mixture since the air control valve will be kept wide open.